Investigating the Relative Stabilities and Electronic Properties of Small

Article pubs.acs.org/JPCA

Investigating the Relative Stabilities and Electronic Properties of Small Zinc Oxide Clusters K. Don Dasitha Gunaratne,†,§ Cuneyt Berkdemir,†,‡,§ Christopher L. Harmon,† and A. W. Castleman, Jr.*,†,‡ †

Department of Chemistry and ‡Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: We report a combined experimental and theoretical study investigating small zinc oxide clusters. A laser vaporization source and a time-of-flight (TOF) mass spectrometer are employed to produce and identify anionic clusters in the ZnnOm (n = 1−6, m = 1−7) size regime. The adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of Zn3O3− and Zn3O4− clusters are determined via anion photoelectron spectroscopy. We have utilized density functional theory (DFT) calculations to explore the possible geometries of neutral and anionic Zn3Om (m = 3−5) clusters, while the theoretical ADE and VDE values are compared with experimental results. The experimentally observed relative abundances among the Zn3Om− (m = 3−5) clusters are investigated through calculations of the detachment energies, dissociation energies, and HOMO−LUMO gaps. We find that the Zn3O3 cluster maintains enhanced stability compared to their oxygen-rich counterparts. Furthermore, by coupling the experimentally obtained photoelectron angular distributions of Zn3O3− and Zn3O4− with electronic structure calculations, the nature of the highest occupied molecular orbitals is discussed, with the goal of aiding the isolation (ligand-capped)/deposition of these building blocks.

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INTRODUCTION One of the key driving factors in the field of clusters and nanomaterials is the ability to identify building blocks that show promise for pushing the limits of currently available technology. Identifying these species and observing the change in properties upon varying a single atom contributes to the knowledge of building cluster-assembled materials, which is a viable alternative to the bottom-up and top-down approaches commonly used.1,2 Particularly, the formation, growth, and shape control of clusters and nanomaterials have been reported in the synthesis of metals, semiconductors, and metal oxides,3−6 which have been of great interest with respect to potential applications such as electronic, photonic, and optoelectronic devices.7,8 One such material that has been subjected to numerous investigations in the condensed phase,9−15 as well as in the gas phase, is zinc oxide (ZnO).16−18 Due to its ability to absorb ultraviolet light (a direct wide band gap semiconductor with a band gap of 3.4 eV at 300 K19), it has already found a niche in various industries, including sunscreen and paint. Additionally, ZnO-based materials have been used as replacements for GaN-based structures in optoelectronics, radiation hard material for electronic devices, transparent material for electronic circuits, dilute-magnetic semiconductors, and ferromagnetic material for spintronics.20,21 Previous transition-metal oxide studies conducted in our group had explored the formation22 and reactivity of metal oxides,23−25 as well as the effects of strong-field ionization of metal oxide clusters26 in metals other than zinc. Gas-phase experiments involving zinc atoms are possibly limited by the © XXXX American Chemical Society

immeasurably low electron affinity and high ionization potential27 due to having completely filled subshells in the neutral electronic configuration28 and therefore a low tendency to accept/donate/share electrons from/to/with other atoms, molecules, or ligands. However, due to the presence of oxygen, the properties of the metal are significantly altered, and ZnO materials inherit semiconducting properties. A previous photoelectron spectroscopic study by Wang et al. investigated the ground and excited states of the ZnO diatomic.29 A comprehensive review by Ozgur et al.19 details the work leading to the current knowledge of the properties of ZnObased materials and devices. Considerable theoretical efforts also have been devoted to characterizing the geometric structures and the electronic properties of ZnO clusters.30−34 Matxain et al.30 studied the ground-state geometries and energies of small (ZnO)n (n = 1− 9) clusters and predicted ring-like configurations to be the global minima for the clusters with n ≤ 7, whereas threedimensional spheroidal structures became more favorable for the larger clusters (n = 8, 9). Moreover, it was emphasized that Zn2O2 and Zn3O3 ring clusters would be useful as a building block of cluster-assembled materials. Mingwen et al.31 investigated the energetics, stable configurations, and electronic structures of the (ZnO)n clusters (n = 9−64) by using firstprinciples calculations. Reber et al.32 calculated the geometries Received: March 27, 2012 Revised: November 29, 2012

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dx.doi.org/10.1021/jp3029374 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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functional with the Lee−Yang−Parr correlation functional (B3LYP)40 was chosen for taking into account the exchange and correlation effects. The “tight” and “qc” keywords were used in all instances because the self-consistent calculations (SCF) of the electronic structures often showed problematic convergence, especially for higher spin multiplicity calculations. The geometry optimization was performed without any symmetry restriction, and the frequency calculations were conducted on all geometries. The zero-point energy (ZPE) correction was included in the total energy of clusters, and its value was around 0.23−0.45 eV. To demonstrate the applicability of the DFT methods and basis sets that we selected for this study, a test calculation was performed to obtain the bond length and vibrational frequencies of the ZnO and ZnO− clusters for both PBEPBE41−43 and B3LYP methods with 6-311++G(3d),39 Aug-cc-pvQZ,44 and LanL2DZ45−48 basis sets. We note that the results with the B3LYP/6-311+ +G(3d) level of theory were in good agreement with experimental values. The adiabatic detachment energy (ADE) and vertical detachment energy (VDE) results for the lowestenergy structures were further refined using the coupled cluster singles and doubles approach including the effect of unlinked triples (CCSD(T)) approach,49 which was determined using perturbation theory as implemented in Gaussian 09. In the CCSD(T) calculation, the optimized lowest-energy structures determined from the B3LYP/6-311++G(3d) level of theory were recalled, and the total energy calculations were performed without making geometry optimization at the CCSD(T)/6311++G(3d) level of theory. In all instances, the unrestricted (open-shell) variation of the handling electron spin was utilized.

of (ZnO)n (n = 2−18, 21) clusters and showed that the transition from a unit of ZnO to the bulk wurtzite ZnO tends to adopt hollow ring, tower, and cage structures. Al-Sunaidi et al.33 reported the stable and low-energy metastable structures of (ZnO)n clusters (n = 1−32). Although previous experimental and theoretical studies have been conducted on these systems, there is still a lack of a systematic investigation of oxygen-rich zinc clusters, and to the best of our knowledge, the Zn3Om− (m = 3−5) clusters have not been investigated theoretically nor experimentally. This paper presents our work on the gas-phase production of anionic zinc oxide clusters in the ZnnOm− (n = 1−6, m = 1−7) size regime and reveals the photoelectron spectra of the Zn3O3− and Zn3O4− clusters. Moreover, utilizing density functional theory (DFT), we have explored the geometries and electronic properties of neutral and anionic Zn3Om (m = 3−5) in order to identify stable species and to better understand the photoelectron angular distributions obtained through experiment. Identifying stable clusters and understanding their valence electronic structure are important steps toward isolating/ depositing these basic building blocks in the condensed phase.

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EXPERIMENTAL DETAILS The experiments were conducted in an apparatus that incorporated a laser vaporization source and a Wiley−McLaren time-of-flight (TOF) mass spectrometer. The methods have been described in detail previously,35 and hence, only a brief overview is presented. The ZnnOm anions are formed in a laser vaporization (LaVa) source with a buffer gas of helium maintained at a backing pressure of 3−4 atm. The ablation laser (532 nm Nd:YAG; 10 Hz) is focused on a translating and rotating zinc oxide rod (PVD Materials Corp. Wayne, NJ, U.S.A.). The key aspect of forming the clusters is the timing of the introduction of the buffer gas to the ablation plasma within the source. A DG-535 digital delay generator (Stanford Research Systems, Inc. Sunnyvale, CA, U.S.A.) enables precision timing control of the laser pulse, backing gas pulsed valve, and the Wiley−McLaren TOF grid assembly voltage pulse. The neutral and charged species produced are supersonically expanded into vacuum, and the molecular beam is skimmed before entering the TOF region from which anions are perpendicularly extracted toward the microchannel plate detector (MCP, Photonis USA Inc., Sturbridge, MA, U.S.A.). The anion beam can be further controlled by the einzel lens assembly at the beginning of the 1.8 m TOF tube. This enables spatial focusing of the anion molecular beam. The anion signal is collected by employing a data acquisition software (Gagescope 12100) and averaged over 500 laser shots per mass range. When the mass peak of interest is identified, the photodetachment laser pulse is intersected with the anion mass packet. If sufficient photon energy is available, the electrons are photodetached and instantaneously guided to the MCP and phosphor screen by the velocity map imaging assembly.36 The images impinged on the phosphor screen are recorded by a charge-coupled device camera. The photoelectron angular distribution and kinetic energy are extracted using the BASEX37 reconstruction algorithm, as described previously.38

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RESULTS AND DISCUSSION A. Relative Abundance and Photoelectron Spectroscopy of Select Clusters. The mass distributions of the ZnnOm− (n = 1−6, m = 1−7) clusters are presented in Figure 1. The top distribution presents the 75−520 m/z range, while the middle (75−300 m/z) and bottom (300−520 m/z) mass distributions provide more details of the total m/z range. Observing the smaller mass range (Figure 1, middle), the

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Figure 1. Mass spectrum of anion ZnO-based clusters in the ZnnOm− (n = 1−6, m = 1−7) size regime; full spectrum (top), 70−300 m/z (middle), 300−520 m/z (bottom). The red arrows depict the disappearance of the anion ZnnOn+2 (n = 5, 6) clusters at higher masses.

THEORETICAL METHODS AND COMPUTATIONAL DETAILS All calculations carried out in this study were performed with the Gaussian 09 program.39 The Becke three-parameter hybrid B



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prominent clusters are seen to have the composition, ZnnOm− (m = n, n+1, n+2) (e.g., Zn2O2−, Zn2O3−, and Zn2O4−). Of these, ZnnOn+1− clusters stand out as the more abundant species. A low-intensity signal of Zn2O− is observed at around 144 m/z. Although we assume that the obtained mass spectrum would consist of mainly pure zinc oxide species, we cautiously note that the observed ZnnOm− (n = 1−6, m = 1−7) cluster distribution could be affected by the formation of hydrides due to inherent hydrogen contamination within the apparatus. The electron binding energies (eBE) of the Zn3O3− and Zn3O4− species (See Figure 2) were obtained by the velocity

preserves the higher-intensity features in the photoelectron spectrum while reducing the lower-intensity noise. The ADE and VDE of the Zn3O3− and Zn3O4− cluster’s neutral ← anion transitions have been determined from the electron binding energy spectra in Figure 2. The beginning of the onset of the peak is used to obtain the ADE,50 while the highest point in the photoelectron spectrum is taken as the VDE. The ADE and VDE of Zn3O3− are 1.14 and 1.71 eV, and the ADE and VDE of Zn3O4− are 2.02 and 2.28 eV, respectively. As the cluster sizes evolve from Zn4Om− to Zn6Om−, we observe a divergence from the abundance pattern of the smaller clusters. Starting from ZnO, the formation of ZnnOn, ZnnOn+1, and ZnnOn+2 anionic clusters is presented in Figure 1. The Zn4O4, Zn4O5, and Zn4O6 anionic clusters have similar abundances as the smaller clusters, but when approaching a cluster with five zinc atoms, only the Zn5O5 and Zn5O6 anionic species are formed with significant abundance. Similarly, Zn6O6 and Zn6O7 anionic clusters are formed, while the cluster with one more oxygen atom (Zn6O8) does not have the relative abundance pattern observed in the clusters comprised of 1−4 zinc atoms. These two incidents are noted by the red arrow on the top mass spectrum of Figure 1. This may be due to a zinc oxide anionic cluster with a preferred structure gaining dominance in this size regime. The relative abundances of ZnnOn+1 clusters and the diminishing signal of ZnnOn+2 clusters may provide insight into the growth and basic building blocks of larger ZnO-based systems. While it is of interest to explore the growth channels of zinc oxide clusters as they build up to larger clusters, it is beyond the scope of the current study, and further investigations are warranted to examine the geometry and energy minima that lead to this experimentally observed change in cluster formation. In order to obtain a more coherent understanding of the stability and electronic properties of small zinc oxide clusters, we have performed a theoretical analysis that explores geometries as well as the electronic structure of neutral and anionic Zn3Om (m = 3−5). This analysis would shed light on whether the obtained mass intensity distribution is a good representative of the relative stability of the individual clusters or whether the availability of oxygen in the experiment results in abundances not directly proportional to their stability in the gas phase.

Figure 2. Photoelectron images and electron binding energy (eBE) spectra of Zn3O3− and Zn3O4− clusters obtained at 2.33 eV (532 nm) photon energy.

map imaging method.36 The velocity map imaging experiment requires that the photodetachment laser pulse be polarized in the plane of the detector to aid in the reconstruction process of converting a two-dimensional angular distribution image, that impinges on the electron detectors and phosphor screen, to a 3D electron cloud.37 A center slice of this reconstructed Newton sphere reveals the electron binding energy spectra depicted in Figure 2. This polarization requirement affects the photodetached electron signal because only the anions with transition dipole moments aligned favorably with the laser pulse will undergo the electronic transitions, provided that the photon energy overcomes the electron binding energy of the anion. Gaussian smoothing has been implemented in order to reduce the noise generated in the captured photoelectron image; this

Table 1. Comparison of Computed Results and Previous Experimental Data of Diatomic ZnO and ZnO− Clustersa ZnO− 6-311++G(3d) methods 2

B3LYP 2 PBEPBE

Aug-cc-pvQZ

1

B3LYP 1 PBEPBE

experimental

ω

R

ω

R

ω

R

ω

1.776 1.764

614 658

1.775 1.762

614 657 ZnO

1.865 1.855

518 559

1.787

625 ± 40

6311++G(3d) methods

LanL2DZ

R

Aug-cc-pvQZ

LanLD2DZ

experimental

R

ω

EA

R

ω

EA

R

ω

EA

R

ω

EA

1.713 1.707

747 762

2.283 2.157

1.709 1.702

748 763

2.244 2.165

1.763 1.764

699 711

1.739 1.775

1.719b

805 ± 40

2.088

R (in Å) and ω (in cm−1) parameters represent the bond length and the vibrational frequency, respectively. EA (in eV) is the abbreviation of electron affinity, which represents the difference between the total energies of the neutral and anion ZnO clusters at their respective optimized geometries. Experimental data are taken from ref 52. The superscripts that are placed on the left-hand side of DFT methods show the spin multiplicity (M) values of anion and neutral ground states. The ZPE correction is included in the EA calculation. bThe result of the CCSD(T) calculation is from ref 53. a

C



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Figure 3. The optimized structures of (a) Zn3O3, (c) Zn3O4, and (e) Zn3O5 neutral and (b) Zn3O3−, (d) Zn3O4−, and (f) Zn3O5− anion clusters at the B3LYP/6-311++G(3d) level of theory. The ground-state structures corresponding to the different isomeric Zn3Om and Zn3Om− (m = 3−5) clusters and some representative low-lying isomers are ordered according to their relative energies. ΔE is the relative energy (in eV) with respect to the corresponding ground state. The gray and red balls represent zinc and oxygen atoms, respectively. The bond lengths are in units of Å. Note that the spin multiplicity (M) is shown as well.

neutral ZnO (ω = 81151 and 805 ± 4052 cm−1). The optimized bond length and vibrational frequency of ZnO−, which were calculated at the B3LYP/6-311++G(3d) and B3LYP/Aug-ccpvQZ levels of theory, are in good agreement with previous experimental results because the deviations are within 0.01 Å for the bond length and also in the range of the measured vibrational frequencies. However, in the case of the LanL2DZ basis set, computational bond lengths are about 0.08 Å longer for both B3LYP and PBEPBE methods, and the vibrational frequencies are too small compared to the experimental results. Thus, from the methods and basis sets utilized to investigate geometry in this study, the B3LYP/6-311++G(3d) and B3LYP/Aug-cc-pvQZ levels of theory produce the results that

B. Theoretical Investigation on Neutral and Anionic Zn3Om (m = 3−5) Clusters. Choosing an Appropriate Method and Basis Set. To interpret the observed relative mass spectrometric abundances and the measured PES of Zn3O3− and Zn3O4− clusters, we carried out DFT electronic structure calculations for both Zn3Om and Zn3Om− (m = 3−5) clusters. In advance, to establish the accuracy of our calculations, we present results on anion and neutral diatomic ZnO, which are compared to existing experimental data51,52 (see Table 1). Recently measured photoelectron detachment spectra of the diatomic ZnO showed that the bond length of the anion was 0.07 Å longer than that of the neutral.52 Consistent with the longer bond length, the harmonic vibrational frequency of ZnO− (ω = 625 ± 40 cm−1) was smaller than that of the D



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Geometry of Zn3O3 and Zn3O3− Clusters. As shown in Figure 3a, the lowest-energy structure of the Zn3O3 cluster is a hexagonal planar ring composed with alternating Zn and O atoms with bond lengths of 1.823 and 2.691 Å between Zn−O and Zn−Zn bonds, respectively. These values are in good agreement with previously published theoretical results.32,55,56 Although our interest is mainly in the lowest-energy structures of Zn3Om and Zn3Om− (m = 3−5) clusters, structures of lowlying isomers are presented because it is probable that they may be formed in our laser vaporization source as well. To achieve this end, the zigzag chain and the rhombus-plus chain structures of the Zn3O3 cluster were selected to compare their energetic stabilities with the ring-shaped Zn3O3 cluster. The ring-shaped Zn3O3 cluster is seen to be 1.87 and 2.88 eV more stable than the zigzag chain structure and the rhombus-plus chain structure, respectively. This comparison is consistent with the finding of Wang et al.56 as well. For the anion Zn3O3− cluster, we have taken into account the same initial geometries which were used for optimizing the ring-shaped, zigzag chain, and rhombus-plus chain structures of the Zn3O3 cluster. The Zn−O bond lengths of the Zn3O3− cluster are elongated by 0.016−0.030 Å compared to the neutral counterpart. The lengthened Zn−O bonds in the Zn3O3− cluster are accompanied by the elongation of the Zn−Zn bond length by 0.084−0.131 Å (Figure 3b). The relative energies of the zigzag chain and rhombus-plus chain structures with respect to the ring-shaped structure of the Zn3O3− cluster are calculated to be 0.83 and 1.71 eV higher, respectively. This means that the ring-shaped Zn3O3− cluster, much like its neutral counterpart, is more stable than the other isomers considered in this study. Geometry of Zn3O4 and Zn3O4− Clusters. The optimized ground-state structures and selected low-lying isomers of Zn3O4 and Zn3O4− clusters at the B3LYP/6-311G(3d) level of theory are presented in Figure 3c and d, respectively. The geometry optimizations are obtained by a sequential oxidation process of the ring-shaped, zigzag chain, and rhombus-plus chain structures of Zn3O3 and Zn3O3− clusters, considering different spin multiplicities. More specifically, the lowest-energy structure of the Zn3O4 cluster is formed by adding an additional oxygen atom into one of the O-bridged sites of the ring-shaped Zn3O3 cluster, as shown in Figure 3c. One of the low-lying isomers of the Zn3O4 cluster is formed by attaching an oxygen atom to the zinc atom that is placed at the edge of the zigzag chain structure of the Zn3O3 cluster, shown in Figure 3c. This isomer is only 0.29 eV less stable than the lowestenergy structure of the Zn3O4 cluster. Another isomer is obtained by modifying the rhombus-plus chain structure of the Zn3O3 cluster with an additional oxygen atom, but its relative energy is calculated to be 2.59 eV higher than that of the lowest-energy structure of the Zn3O4 cluster. As for the Zn3O4− cluster, similar oxidation processes are applied to find the lowest-energy structure (Figure 3d). It is interesting to note that the zigzag chain structure of the Zn3O4− cluster has a remarkable stability because its relative energy is 1.19 and 2.29 eV less than that of other isomers, as indicated in Figure 3d. Geometry of Zn3O5 and Zn3O5− Clusters. Finally, the lowest-energy structure of the Zn3O5 cluster is obtained by adding two oxygen atoms into the ring-shaped Zn3O3 cluster. Zn3O5 has two nearly degenerate isomers, which are shown with relative energies of ΔE = 0 and 0.56 eV in Figure 3e. Specifically, an additional oxygen atom is inserted into a Zn−O bond of the ring-shaped Zn3O3 cluster, forming the lowestenergy structure of the Zn3O4 cluster with the grouping of two

are in good agreement with previous experiment and theory.53,54 As for the electron affinity (EA) comparison at the relevant levels of theory, with the experimental data given in Table 1, mean deviations of computed EAs from the experimental value are 0.19, 0.17, and 0.35 eV at the B3LYP/6-311++G(3d), B3LYP/Aug-cc-pvQZ, and B3LYP/LanL2DZ levels of theory, respectively. This deviation is 0.17, 0.18, and 0.31 eV at the PBEPBE/6-311++G(3d), PBEPBE/Aug-cc-pvQZ, and PBEPBE/LanL2DZ levels of theory, respectively. The B3LYP and PBEPBE methods together with 6-311++G(3d) and Augcc-pvQZ basis sets overestimate the experimental value of EA, whereas both methods together with the LanL2DZ basis set underestimate it. Even if 6-311++G(3d) or Aug-cc-pvQZ basis sets for both methods provide the smallest error with respect to the experimental value of EA, the use of the Aug-cc-pvQZ basis set for both methods has dramatically increased the computational time while not giving significantly better results compared to the 6-311++G(3d) basis set. On the basis of the results given in the test calculation for ZnO and ZnO− clusters, the B3LYP/6-311++G(3d) level of theory was chosen as ideal for our calculations because it exhibits good performance on optimized geometries and vibrational frequencies as well as reasonably good performance on electron affinities. In Table 1, spin multiplicity (M) results obtained for the different methods and basis sets are presented. Although the 3Π state of the ZnO diatomic is about 0.07 eV below the 1Σ+ state with the B3LYP method for all basis sets (Table 1), further refinements using the CCSD(T) method at the B3LYPoptimized geometries show that the 1Σ+ state is about 0.18 eV below the 3Π state. On the basis of this result, the M value of ZnO is determined to be 1, which indicates the 1Σ+ state to be the ground state for diatomic ZnO, which is in agreement with previous results.53,54 The lowest-energy structures and some representative lowlying isomers obtained for Zn3Om and Zn3Om− (m = 3−5) clusters are presented in Figure 3. An extensive search for the possible geometric structures of Zn3Om and Zn3Om− (m = 3− 5) clusters was performed for the singlet, triplet, and quintet states of Zn3Om and the doublet, quartet, and sextet states of Zn3Om− clusters at the B3LYP/6-311G(3d) level of theory. The relative energies (ΔE) with respect to the lowest-energy structures and spin multiplicity (M) values are reported as well. As an initial guess to find the lowest-energy structures of Zn3Om and Zn3Om− (m = 3−5) clusters, we kept the ringshaped structure of Zn3O3 that was studied in detail previously by Matxain et al.55 In the study, they generated several starting points for the geometry optimizations using a “simulated annealing approach” to conduct a reliable search of the global minimum structures of ZniOi (i = 1−9) clusters. Additionally, in a study by Wang et al.,56 a number of structures of (ZnO)n (n = 2−18) clusters were investigated by following some “handmade” structural rules, and an empirical genetic algorithm simulation was successfully applied to the geometry optimization of clusters by combining with DFT results. They found that the lowestenergy structure obtained for the Zn3O3 cluster was in the shape of a hexagonal ring which is similar to the finding of Matxain et al.55 This gives credence to our decision to use this ring-shaped Zn3O3 cluster as a valid starting geometry. In our investigation, we utilized the B3LYP/6-311G(3d) level of theory to obtain the most stable geometries of these zinc oxide clusters. E



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clusters are relatively higher than those of O-rich Zn3Om0/− (m = 4, 5) clusters. These results indicate that neutral and anionic Zn3O3 clusters do not easily dissociate into smaller fragments, and they are relatively stable compared to the O-rich clusters. In addition, the DEs drop down sharply with an increasing oxygen-to-zinc ratio. Especially, the neutral and anionic Zn3O5 clusters are unstable because they have either negative or slightly positive DE values. C. Comparison of Electronic Properties. One of the important parameters in characterizing the relative stability of clusters is the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The results in Table 2 reveal that the HUMO−LUMO gaps of neutral Zn3Om clusters are, in general, larger than those of the anionic Zn3Om− clusters. Among the neutral clusters, the planar ring Zn3O3 cluster has the largest HOMO−LUMO gap (4.40 eV), while the O-rich clusters have gaps of 3.99 (Zn3O4) and 4.05 eV (Zn3O5). The relatively larger HOMO−LUMO gap for the Zn3O3 cluster means that it would be less reactive compared to other neutral clusters presented in Table 2. The Zn3O4− cluster shows the smallest HOMO−LUMO gap (0.37 eV), and thus, it would be more reactive compared to the rest of the anionic clusters in Table 2. The large DE and HOMO−LUMO gap of the neutral Zn3O3 cluster suggests that it is stable against dissociation and reactivity when compared to its O-rich counterparts. In order to verify the lowest-energy structure of the Zn3Om0/− (m = 3−5) clusters, as well as the low-lying isomers, we compared the computed results with the experimental ADE and VDE results (Table 3). As we know, the agreement

oxygen atoms. For the lowest-energy structure of the Zn3O5 cluster, another oxygen atom is added into the O2 site of the lowest-energy structure of the Zn3O4 cluster, and then, an O3 unit is formed. Hence, two nearly degenerate isomers of the Zn3O5 cluster are obtained, as shown in Figure 3e. One of these isomers (ΔE = 0) is found to be at least 0.56 eV more favorable than the other one. An extensive search indicated that the zigzag chain structure of the Zn3O5 cluster, which is formed by a sequential oxidation process of the zigzag chain structure of the Zn3O3 cluster, is only 0.02 eV higher in relative energy than the lowest-energy structure of the Zn3O5 cluster. The lowestenergy structure of Zn3O5− and two low-lying isomers are shown in Figure 3f. The lowest-energy structure of the Zn3O5− cluster (ΔE = 0) has two O atoms individually bonded to Zn atoms of the most stable Zn3O3− cluster. This structure shows remarkable stability compared to the other low-lying isomers of Zn3O5− clusters. As for the low-lying isomers of the Zn3O5− cluster, two additional oxygen atoms are connected to the edge atoms of the zigzag chain structure of the Zn3O3− cluster. This isomer is only 0.09 eV less stable than the lowest-energy structure of the Zn3O5− cluster. The other isomer, which is depicted at the bottom of Figure 3f, is an energetically less favorable structure (ΔE = 0.87 eV), which contains an O3 unit. Analysis of Fragmentation Channels. In order to predict relative stabilities for the Zn3Om and Zn3Om− (m = 3−5) clusters, the fragmentation channels and the dissociation energies (DEs) provide useful information because the clusters that are formed as fragmentation products are likely to be more stable. Moreover, investigating the stability can provide insight into viable pathways for synthesizing cluster-assembled material from potential building blocks.57 In Table 2, we evaluate the

Table 3. Experimental and Theoretical ADEs and VDEs of Lowest-Energy Structures and Low-Lying Isomers of Anionic Zn3Om (m = 3−5) Clustersa

Table 2. Most Favorable Fragmentation Channels, Dissociation Energies (DEs, in eV), and HOMO−LUMO gaps (in eV) of the Most Stable Zn3Om0/− (m = 3−5) Clustersa

CCSD(T)// B3LYP

experimental

cluster

fragmentation channels

DE

HL gap

cluster

ΔE

ADE

VDE

ADE

VDE

ADE

VDE

Zn3O3 Zn3O4 Zn3O5 Zn3O3−

Zn2O2 + ZnO Zn3O3 + O Zn3O3 + O2 Zn2O2− + ZnO Zn2O2 + ZnO− Zn3O3− + O Zn3O3− + O2

5.96 1.93 −0.24 5.44 5.03 3.93 0.53

4.40 4.05 2.82 1.16

Zn3O3−

1.18 2.15 2.71 1.63 1.54 1.90

1.27 2.43 2.90 2.09 1.64 2.27 3.56 2.14 2.08

2.02

2.28

Zn3O5−

1.58 2.63 2.91 3.07 1.97 3.35 4.57 2.86 2.83

1.71

0.37 3.26

1.50 2.58 2.73 3.01 1.49 3.00

1.14

Zn3O4−

0.00 0.83 1.71 0.00 1.19 2.29 0.00 0.09 0.87

Zn3O4− Zn3O5− a

B3LYP

The ZPE correction is included in the DE calculation.

Note that the experimental ADE and the VDE values of the Zn3O5− cluster are not reported because its eBE is higher than the photon energy used for this experiment. The ADE values for Zn3O5− have not been estimated due to the lack of low-lying neutral isomers with similar geometry. The CCSD(T)/6-311++G(3d) level of theory is used to refine the total energy of the ground-state structures that were geometry-optimized with the B3LYP/6-311++G(3d) level of theory. Experimental error limits are within ±0.02 eV of the reported ADE/ VDE values. a

fragmentation channels and the DEs of the clusters in their lowest-energy structures. The DE is described as follows: DE = E T(Zn3O30/ −) + E T(O1,2 ) − E T(Zn3Om 0/ −) (m = 4, 5)

(1)

ET(Zn3O30/−),

where ET(O1,2), and ET(Zn3Om0/−) represent the total energies of the most stable Zn3O30/− clusters, either the O or O2 molecule, and Zn3Om0/− (m = 4, 5) clusters, respectively. Note that the fragmentation channels of Zn3O3 and Zn3O3− clusters will be different than those of others, which are given in eq 1, that is, Zn3O30/− clusters dissociate into Zn2O20/− and ZnO fragments.58 We can consider clusters with large positive DEs to be more stable and those with slightly positive or even negative DEs to be unstable as they are more likely to dissociate. As shown in Table 2, the DEs of Zn3O30/−

between experiment and theory is evident, giving us confidence in our calculated structures. The calculated VDE value of an extra electron from an anion is obtained by taking the total energy difference between an anion and its neutral parent at the anion geometry. The ADE is calculated as the difference between total energies of a neutral and the corresponding anion at their respective ground-state geometries. The ADE and VDE F



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the HOMO states of Zn3O4− and Zn3O5− possess dominant pcharacter. Recently, Ard et al.60 reported successfully capturing ligandcoated vanadium oxide clusters that were abundant in the gas phase employing a laser vaporization flow reactor, where a laser vaporization source generates the clusters that react with the capping ligands as they travel through the flow tube. The products were then collected via a liquid nitrogen trap and characterized by time-of-flight mass spectrometry and Fourier transform infrared spectrometry. The abundant core units of these zinc oxide clusters might be brought into the solution phase and characterized using similar methods. Probing the HOMO of Zn 3O 3 − and Zn3 O4 − using experiment and theory provides insight into the electronic structure of these clusters that have not been previously investigated using photoelectron spectroscopy. From our combined experimental and theoretical results, it is evident that there are fundamental differences in the electronic structure of Zn3O3− and Zn3O4−. These electronic structure differences may prove to be useful when choosing ligands for condensed-phase isolation, as well as for surface preparation in deposition experiments. The challenges of isolating these predicted building blocks involve many factors such as hydride contamination (and its effects on the electronic structure),61 obtaining sufficient mass intensity for probing in the gas phase, as well as obtaining sufficient yield when scaling up to ligation, deposition experiments.

values were computed using the B3LYP/6-311++G(3d) as well as the CCSD(T)/6-311++G(3d) level of theory. However, in the CCSD(T) calculations, we kept the ground-state geometries that were optimized at the B3LYP/6-311++G(3d) level of theory and calculated the total energy values of the relevant clusters by including the ZPE corrections. The 532 nm (2.33 eV) photoelectron spectrum of Zn3O3− reveals the ADE and VDE to be 1.14 and 1.71 eV, respectively. According to our results, the ADE value of Zn3O3− is overestimated at the B3LYP/6-311++G(3d) level of theory, while the refined value calculated by the CCSD(T)/6-311+ +G(3d) level of theory agrees with experiment. The VDE of Zn3O3− calculated by the B3LYP method is consistent with the experimental value, while the value obtained by the CCSD(T) energy refinement is slightly lower. For the low-lying isomers of the Zn3O3− cluster, the computed ADE and VDE values are higher than the obtained value in our experiment, supporting the fact that the species that underwent photodetachment was indeed the lowest-energy structure. For the Zn3O4− cluster, the ADE and VDE results of the CCSD(T) energy refinements show partial agreement with the experimental ADE and VDE, while in the case of B3LYP, the results are relatively higher. Interestingly, one of the isomers (ΔE = 2.29 eV) computed for the Zn3O4− cluster possesses ADE and VDE results that are in good agreement with the experiment. This finding indicates that we are most likely photodetaching electrons from this isomer rather than the lowest-energy structure of the Zn3O4− cluster. Although we have not obtained the PES of the Zn3O5− due to insufficient photon energy (2.33 eV) for photodetachment from this cluster, we included the computational results that would need to be compared with future experimental investigation. On the basis of the computational results that are obtained by the CCSD(T) method at the geometryoptimized B3LYP/6-311++G(3d) level of theory, we estimate the VDE of the Zn3O5− cluster to be 3.56 eV. The ADE values for Zn3O5− have not been estimated due to the lack of lowlying neutral isomers with similar geometry. D. Photoelectron Angular Distribution and Molecular Orbital Analysis. The photoelectron angular distribution of Zn3O3− was obtained with a β value of 0.9 ± 0.2. As discussed previously,59 β values around 1−2 represent photodetachment from σ-molecular orbitals. Higher β values also signify a high percentage of atomic s-electron contribution to the HOMO. Table 4 presents the percentages of s, p, d, and f atomic compositions of molecular orbitals for the lowest-energy structures of Zn3Om− (m = 3−5) clusters obtained by the current level of theory. This reveals that, in the case of Zn3O3−, the observed electronic band (Figure 2) resulted from electron detachment from a HOMO that is predominantly composed of s-electrons with a partial atomic p-electron contribution, while

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CONCLUSIONS In this paper, we have reported observation of the mass spectrum of anionic zinc oxide clusters in the ZnnOm (n = 1−6, m = 1−7) size regime and obtained the photoelectron spectra of Zn3Om− (m = 3, 4) clusters via photoelectron imaging spectroscopy. We have also carried out extensive DFT calculations to predict the ground-state geometries and stabilities of Zn3Om and Zn3Om− (m = 3−5) clusters. The experimental ADE and VDE values are determined to be 1.14 and 1.71 eV for Zn3O3− and 2.02 and 2.28 eV for Zn3O4−, respectively. These results are consistent with the computed results that are calculated by the B3LYP/6-311++G(3d) level of theory as well as the CCSD(T) method at the B3LYPoptimized ground-state geometries. Our calculations demonstrate that all of the lowest-energy neutral and anion clusters are predicted to be singlet (1A) and doublet (2A) states, respectively. According to the dissociation energy and HOMO−LUMO gap calculations, the Zn3O3 cluster should be remarkably more stable than its O-rich counterparts. It is worth noting that the relative abundances observed in the mass spectrum for Zn3Om− (m = 3−5) clusters were not indicative of the relative stabilities of the neutral or anionic clusters deduced from our theoretical results. Further experimental and theoretical investigations are warranted to explore the cluster growth and stability patterns observed when moving toward larger zinc oxide clusters. The HOMOs of Zn3Om− (m = 3−5) were analyzed, while the photoelectron angular distribution of the electrons photodetached from Zn3O3− was reported to have a β value of 0.9 ± 0.2. This analysis revealed that the HOMO Zn3O3− has a majority of atomic s-electron contribution in the formation of the molecular orbital, while the atomic p-electron contribution dominates the HOMO of the O-rich clusters. These differences in electronic structure of the HOMO may prove to be

Table 4. Atomic Orbital Compositions of the HOMOs Are Shown for Zn3Om− (m = 3−5) Clustersa cluster Zn3O3−

a

Zn3O4−

Zn3O5−

atomic composition

Zn

O

Zn

O

Zn

O

s% p% d%

65 13 2

14 6 0

0 3 6

0 91 0

0 0 10

0 90 0

The f% is not included because of its minor contributions. G



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instrumental in isolating these clusters by ligand-capping or by depositing on premodified surfaces.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

Authors contributed equally to this study.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding from the U.S. Department of the Army (MURI) Grant W911NF-06-1-0280. The computations have been supported in part by the Materials Simulation Center and the Penn State Center for Nanoscale Science (MRSEC-NSF).

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Investigating the Relative Stabilities and Electronic Properties of Small

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